chapter 8: overview: the energy of life
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Overview: The Energy of Life
y The living cell is a miniature factory where thousands of reactions occur and convertsenergy in many ways
An organisms metabolism transforms matter and energy, subject to the laws of
thermodynamics
y Metabolism- all of an organisms chemical reactionsOrganization of the Chemistry of Life into Metabolic Pathways
y Metabolic pathway- begins w/ a specific molecule which is then altered in a series ofdefined steps, resulting in a product.
y Catabolic pathways- break down pathway,release energyy Anabolic pathways- consume energy to build complicated molecules from simpler ones
Forms of Energy
y Energy- the capacity to cause changey Kinetic energy- energy associated w/ the relative motion of objectsy Heat or thermal energy is kinetic energy associated w/ the random movement of atoms
or molecules
y Potential energy- is energy that matter possesses b/c of its location or structurey Chemical energy- the potential energy available for release in a chemical reactiony Energy can be converted from one form to another:
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The Laws of Energy Transformation
y Thermodynamics- the study of energy transformations that occur in a collection ofmatter
y According to the first law of thermodynamics- energy can be transferred andtransformed, but it cannot be created or destroyed(principle of conservation of energy)
y According to the second law of thermodynamics- every energy transfer oftransformation increases the entropy of the universe
y Entropy- a measure of disorder, or randomnessy For a process to occur spontaneously, it must increase the entropy of the universe
Biological Order and Disorder
y Universe- the system plus its surroundingsThe free-energy change of a reaction tells us whether the reaction occurs spontaneously
Free- Energy Change, (G
y Free energy- measures the portion of a systems energy that can perform work whentemp and press. are uniform, as in a cell
y The change in free energy, Gduring a biological process, can be calculated for anyspecific chemical reaction w/ this formula:
G = H TS
y H symbolizes the change in the systems enthalpy(= total energy), S is the change inthe systems entropy, and T is the absolute temp on Kelvin(K)
Free Energy, Stability, and Equilibrium
y Organisms live at the expense offree energyy During a spontaneous change free energy decreases and the stability of a system
increases
y Equilibrium- term for a state of maximum stability
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Free Energy and Metabolism
Exergonic andEndergonic ReactionsinMetabolism
y Exergonic reaction- proceeds with a net release of free energy and is spontaneous
y Endergonic reaction- absorbs free energy from its surroundings and is nonspontaneous
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Equilibrium and Metabolism
y Reactions in a closed system eventually reach equilibrium and can then do work
y Cells in our body- experience a constant flow of materials in and out, preventingmetabolic pathways from reaching equilibrium
y An analogy for cellularrespiration:
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ATP powers cellular work by coupling exergonic reactions to endergonic reactions
y A cell does three main kinds of work Mechanical- such as the beating of cilia Transport- pumping of substances across membranes C
hemical-
the pushing of endergonic reactionsy Energy coupling- the use of an exergonic reaction to drive an endergonic oneThe Structure and Hydrolysis ofATP
y ATP (adenosine triphosphate)- cells energy shuttle, provides energy for cellularfunctions
y Energy is released from ATP when the terminal phosphate bond is broken:
y ATP hydrolysis can be coupled to other reactions:
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How ATP Performs Work
y ATP drives endergonic reactions by phosphorylation, transferring a phosphate to othermolecules
y The three types of cellular work are powered by the hydrolysis of ATP:
The Regeneration ofATP
y Catabolic pathways drive the regeneration of ATP from ADP and phosphate:
Enzymes speed up metabolic reactions by lowering energy barriers
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y A catalyst is a chemical agent that speeds up a reaction without being consumed by thereaction
y An enzyme is a catalytic proteinThe Activation Barrier
yEvery chemical reaction between molecules involves both bond breaking and bondforming
y The activation energy, EA, or free energy of activation- energy required to contort thereactant molecules so the bonds can change, its supplied in form of heat from
surroundings
y The energy profile for an exergonic reaction:
How Enzymes Lower the EA Barrier
y An enzyme catalyzes reactions by lowering the EA barriery The effect of enzymes on reaction rate:
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Substrate Specificity of Enzymes
y Substrate- reactant an enzyme acts ony enzyme-substrate complex formed when the enzyme binds to its substratey A
ctive site-
region on the enzyme where the substrate binds(a)
y Induced fit of a substrate- brings chemical groups of the active site into positions thatenhance their ability to catalyze the chemical reaction(b)
Catalysis in the Enzymes Active Site
y The catalytic cycle of an enzyme:
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y The active site can lower an EA barrier by:o Orienting substrates correctlyo Straining substrate bondso Providing a favorable microenvironmento C
ovalently bonding to the substrateEffects of Local Conditions on Enzyme Activity
Effects of Temperature and pH
y Each enzyme has an optimal temperature in which it can function:
y And an optimal pH in which it can function:
y Cofactors- nonprotein enzyme helpersy Coenzymes- organic cofactors
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y Competitive inhibitors bind to the active site of an enzyme, competing with thesubstrate:
y Noncompetitive inhibitors bind to another part of an enzyme, changing the function
Regulation of enzyme activity helps control metabolism
Allosteric Regulation of Enzymes
y Allosteric regulation- term used to describe any case in which a proteins function atone site is affected by binding of a regulatory molecule at another site
y Most allosterically regulated enzymes are constructed from one or more polypeptidechains
y They change shape when regulatory molecules bind to specific sites, affecting function
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y Cooperativity- a form of allosteric regulation that can amplify enzyme activity
y Feedback inhibition- The end product of a metabolic pathway shuts down the pathway